very long at a
time.

For three weeks, perhaps, the plant may experience drought, and during
this period it takes up no phosphoric acid, and its growth practically
comes to a standstill; but this period of drought is followed by rain
and warm weather, and the plant, if it is to be ripe by harvest-time,
must make up for lost time. It must grow as much the next few days under
these favourable climatic conditions as it would have grown under normal
conditions in double or treble the time. In order to do so, however, it
must be able to obtain plenty of phosphoric acid, and this is only
possible where there is a decided excess of phosphoric acid present in
the soil.

The richness of a soil, therefore, in phosphoric acid, must be such
that it is not only able to supply the ordinary wants of the plant, but
to provide an excess when such an excess will be needed; for one must
remember that the amount of plant-substance formed in the course of a
few days under favourable conditions is very great, and that the amount
consequently of phosphoric acid which plants assimilate during that
period must also be very considerable.

_Method of Application._

In conclusion, as to the method of application of the slag,
agriculturists must be _warned against mixing it with sulphate of
ammonia_; for if this is done, a _considerable loss of ammonia_ will
ensue, set free from the sulphate by the action of the free lime which
the Thomas-slag contains. With nitrate of soda and potash salts it may
be freely mixed. Such mixtures, however, are apt to form themselves into
little balls, which soon become very hard. They should therefore only be
mixed shortly before use. To overcome this difficulty, Professor Wagner
recommends the mixture of a little peat or sawdust with the slag.

FOOTNOTES:

[233] See Appendix, p. 417.

[234] _Vide_ paper on "Basic Slag: Its Formation." By Stead and
Ribsdale. 'Journal of the Iron and Steel Institute,' 1887, p. 230.

[235] _Vide_ Professor Wagner's pamphlet, 'Der Duengewerth und die
rationelle Verwendung der Thomas Schlacke,' Darmstadt, 1888.

[236] No. 1 fineness was such as passed entirely through a fine gauze
sieve of 250 wires to the linear inch.

[237] No. 2 fineness was such as passed entirely through the regular
standard sieve--_i.e._, containing 120 wires to the linear inch.

[238] No. 3 was what would not pass through the standard sieve.

[239] 'Transactions of the Highland and Agricultural